Device-to-Device (D2D) communication in a cellular communications system is receiving a significant amount of interest, particularly with respect to next and future generation networks. D2D communication is communication between a source device and a target device, where both the source device and the target device are wireless devices (e.g., User Equipment devices (UEs) in Third Generation Partnership Project (3GPP) terminology). Some of the potential advantages of D2D communication include off-loading by the cellular network, faster communications, increased awareness of surrounding wireless devices of interest (e.g., running the same or a related application), more reliable and higher quality links due to shorter distances between devices, and the like. Some applications of D2D communication include video streaming, online gaming, media downloading, Peer-to-Peer (P2P), file sharing, and the like.
D2D communications may coexist with cellular communications. Unlike conventional cellular communications, D2D-capable wireless devices that are in proximity to each other can use a “direct mode” or a “locally routed” path for data communication. Thus, a D2D communication can establish a direct local link between wireless devices to bypass conventional cellular communications.
Cellular communications networks based on the 3GPP Long Term Evolution (LTE) standard may include a combination of densely deployed nodes that control respective cells in a heterogeneous network. These networks provide load balancing in response to demands for bandwidth and improved user experience. Mobility operations are performed by network nodes or wireless devices to maintain a uniform user experience in LTE cellular communications. Mobility operations include cell selection, reselection, changing cells, or the like. Mobility operations also include prioritization or ranking of cells, Radio Access Technologies (RATs), Public Land Mobile Networks (PLMNs), or the like.
Cell selection and reselection is typically based on measurements performed at a wireless device. For example, a wireless device may measure a received signal, in Decibels (dB), from a cell. The cell may be selected if it satisfies criteria such as a dB value measured at or above a threshold, or the quality signal value (in dB) is above a threshold. Cell reselection may occur when a wireless device is being served by a cell and a neighboring cell is a threshold value better for a certain period of time compared to the serving cell. Performing cell reselection can also be speed-dependent and consider the speed of only one wireless device for cell selection.
Cell selection can be performed for a wireless device in a particular state or when transitioning to a different state. For example, cell selection may be required upon transition by a wireless device from an idle state to an active (e.g., connected) state. Examples include cell transition from EMM-DETACHED to EMM-REGISTERED, and from ECM-IDLE to ECM-CONNECTED. To select a cell, a wireless device may first identify a cell from each carrier frequency that has the strongest signal. These cells are generally desirable because they provide an improved quality of radio communications due to an improved radio link. The wireless device then identifies a suitable cell as the strongest cell from among those with the strongest signal for each carrier frequency. A wireless device may identify an acceptable cell if the wireless device failed to identify a suitable cell. After a suitable or acceptable cell has been identified, the wireless device camps on that cell and commences performing cell selection or reselection. To be camped on a cell refers to a wireless device (e.g., UE) that has completed the cell selection/reselection process and has chosen a cell. The wireless device monitors system information and (in most cases) paging information.
A suitable cell may have measured attributes that satisfy selection criteria. In addition, the cell may have a designated, registered, or equivalent PLMN, the cell is not barred or reserved, and the cell is not in a forbidden tracking area for roaming. On the other hand, an acceptable cell has measured attributes that merely satisfy selection criteria, and the cell must also not be barred from being selected.
Cell reselection can be performed by a wireless device in an idle state. Cell reselection may be based on reselection criteria that are used to identify a cell that the wireless device should camp on. Similar to selection, reselection involves performing measurements on serving and neighboring cells. A wireless device can generally search and measure cells without having neighbor cell lists. Examples of reselection criteria include intra-frequency reselection based on ranking of cells, and inter-frequency reselection based on absolute priorities. Under inter-frequency reselection, a wireless device tries to camp on the highest priority frequency available. Absolute priorities for reselection are provided only by the Registered PLMN (RPLMN) and are valid only within the RPLMN.
Cell reselection can be performed by a wireless device in a connected state. For example, reselection in LTE RRC_CONNECTED state is a part of a handover procedure. These handovers are network-controlled and wireless device-assisted. That is, the handover decision is made by the network based on wireless device measurement reports and Radio Resource Management (RRM) information such as cell load level.